1. Field of the Invention
The present invention relates to a control of a vehicle automatic transmission which has a multiple stage transmission gear mechanism and in which an appropriate gear stage of the gear mechanism is automatically selected in accordance with the vehicle operating conditions. More specifically, the present invention pertains to an automatic transmission control system which can minimize the gear shift shock.
2. Description of the Prior Art
A vehicle automatic transmission generally includes a transmission gear mechanism having a plurality of gear stages of different gear ratios and an appropriate gear stage is automatically selected in accordance with the vehicle operating conditions. For selecting the gear stage, the transmission gear mechanism is provided with an appropriate number of friction devices such as clutches and/or brakes so that elements of the gear mechanism are selectively constrained by the friction devices to determine the desired gear train in the gear mechanism. The friction devices are provided with hydraulic actuators which are operated through valves controlled by a control unit.
In this type of automatic transmission, it is required to minimize the shift shocks which are produced when the transmission gear mechanism is shifted from one gear stage to another gear stage. The Japanese patent application No. 55-61070 filed on May 7, 1980 and laid open to public inspection on Dec. 3, 1981 under the laid-open No. 56-156543 discloses a transmission control system which is aimed to decrease the transmission shift shock. According to the proposal by the Japanese application, the actuator is provided with a plurality of drain passages having flow restrictions of different sizes. The flow restrictions are controlled so that the flow restriction of the largest size is at first used in the initial stage of actuation of the actuator to thereby move the actuator at a faster rate and sequentially switch to a flow restriction of a smaller size to decrease the rate of movement of the actuator.
It is understood that the control system as proposed by the Japanese application is effective to decrease the shifting shock of the transmission gear mechanism to a certain extent. However, there still is a possibility that a shift shock will be produced due to wear or other changes in the actuators and the friction devices. More specifically, the actuators and the friction devices may be dimensionally different due to manufacturing tolerances and such dimensional differences may cause differences in the conditions wherein the actuators are actuated and the friction devices are engaged. It may be possible to adjust the friction devices during the manufacture of the transmission so that the gear shifting operations are carried out with a minimum gear shift shock. However, even when such adjustments are made when the transmission is manufactured, different conditions are produced through prolonged use of the transmission due to wear and/or change in the adjustment of the friction devices. For example, when the friction device is engaged to shift the transmission gear mechanism from a lower gear stage up to a higher gear stage, the aforementioned wear and/or change in the adjustment of the friction devices through a prolonged use may cause a substantial change in the rate of engagement of the friction devices. In a case where the rate of engagement is increased there will be an increase in the gear shift shock. To the contrary, when the rate of engagement is decreased, there will be a preferable effect on the gear shift shock. However, there will be an increase in the slip of the friction members so that the life of the friction device is decreased.
In some types of automatic transmissions, two gear stages are alternately selected by operating two friction devices. For example, in a transmission having a transmission gear mechanism with four forward gear stages, the shifting operation between the second and third gear stages may be carried out by actuations of a clutch and a brake so that the third gear stage is established by engaging the clutch and disengaging the brake whereas the second gear stage is established by disengaging the clutch and engaging the brake. The hydraulic control system for the transmission is designed so that the clutch and the brake are applied with the hydraulic pressure from the same line in such a manner that pressure for engaging the clutch is used to disengage the brake. Further, the clutch and the brake have a common drain line so that the hydraulic pressure is drained from the drain line to disengage the clutch and engage the brake.
In this type of automatic transmission, it is important to have the operating timings of the clutch and the brake matched with each other. Particularly, there is a strong possibility that a shift shock will be produced when the transmission gear mechanism is shifted down from the third gear stage to the second gear stage. Even when the drain system as taught by the aforementioned Japanese application is adopted, it is difficult to avoid the gear shift shock.
The U.S. Pat. No. 4,283,970 proposes another measure for preventing the gear shift shock in an automatic transmission. According to the proposal by this U.S. patent the hydraulic pressure is appropriately controlled so that the shifting operation is completed in a desirable time period. More specifically, the hydraulic pressure is controlled so that the rate of engagement of the friction device is desirably controlled throughout the time period of bringing the friction device from the disengaged position to the engaged position However, the proposed control is not effective in preventing the gear stage shift shock because no measure is provided for adjusting the rate of engagement of the friction device in the final period of the gear shifting operation.